1. Field of the Invention Field of Invention
The present invention relates to the aqueous synthesis of photoluminescent nanocrystalline quantum dots.
2. Brief Description of the Prior Art
Semiconductor nanocrystalline quantum dots (QDs) with bioconjugates on the surface have been studied extensively because of their unique optical properties. QDs are inorganic nanoparticles that emit light at a specific wavelength when excited by light. When light shines on QDs, electrons in the valence band are excited to the conduction band, forming short-lived (nanoseconds) electron-hole pairs called excitons, which emit photons of a specific wavelength when the electron-hole pairs eventually recombine. The excitonic emission is independent of the wavelength of the excitation light. This makes it easier to excite QDs to luminescence than the traditional fluorescent molecules that require a specific excitation wavelength. The wavelength of the emitted photons of QDs, on the other hand, is specific and can be controlled by the QDs' particle size and composition. The synthesis of QDs was developed mostly in the 1990's. In the last few years, the interest in using QDs in biomedical imaging has exploded due to advances in surface modification of QDs that have made them accessible for antibody immobilization and detection of antibody-antigen binding.
Using QDs as imaging markers inside living organisms is one of the exciting new nanobiotechnologies. QDs can be used as biological markers to find a disease as well as to carry a drug to the exact cell that needs it by immobilizing antibodies on the surface of the QDs. QDs may be specific to a particular disease and may be tailored to bind only to infected cells. Detection may be carried out either by locating the QDs' particles or by detecting signals emanating from the QDs' particles. For example, luminescence of antibody-coated QDs bound to the cancerous tissue in a mouse helped locate the tumor.1 Until now the main biological tags that have been employed are organic fluorophores or radioactive labels.2 Radioactive labels are short lived and radioactive. Concerns about the use of radioactive materials in the body always arise. Organic fluorophores have wide emission spectra and the emission is not as bright as that of QDs. In comparison to conventional dye molecules, QDs have the advantages of having tunable fluorescence signatures, narrow emission spectra, brighter emissions, and good photostability.3 Due to the enormous interest in using QDs as biological tags, QDs are now commercially available from quite a number of companies. However, the complexity of the existing organic-based synthesis route for fabricating commercial QDs makes the price prohibitively high, as much as U.S. $1200/g without bio-conjugation4, and $3200/mg for bioconjugated QDs.5 Part of the complexity of the existing QDs production technology stems from the need to improve the photoluminescence yield by eliminating the broadband emission of earlier QDs by capping with an inorganic layer. Making QDs water-soluble is another challenge for biomedical applications.
Both groups II-VI nanocrystals such as CdSe, CdTe, CdS,6,7 ZnS,8 and ZnSe, and groups III-V nanocrystals such as InP and InAs have been synthesized and studied extensively in the past.9 One type of quantum dot currently on the market is based on CdSe nanocrystals capped by, for example, ZnS. The synthesis follows the method popularized by Bawendi's group at MIT involving the pyrolysis of organometallic precursors, dimethylcadmium and trioctylphosphine oxide (TOPO) to form CdSe nanocrystals. ZnS capping on CdSe was done using diethylzinc and hexamethyldisilathiane precursors.10 
Alivisatos and coworkers further made QDs water-soluble by addition of a silica/siloxane coating.11 With a silica coating, 3-(mercaptopropyl)trimethoxysilane (MPS) is then adsorbed on the nanocrystals and displaces the TOPO molecules, making the surface of the QDs suitable for antibody immobilization.12 These processes are complex involving multiple steps and a change of solvent from organic to aqueous during the process.
An aqueous process for the manufacture of CdS QDs was published recently using adenosine triphosphate (ATP) as the capping molecule.13 This process suffers from the disadvantage that the luminescence spectrum of the resultant CdS QDs includes an undesirable non-excitonic broadband emission between 500 nm to 700 nm wavelength.
Rogach et al.14 describes the synthesis of oxidation-stable CdTe nanoclusters in aqueous solution using 2-mercaptoethanol and 1-thioglycerol as stabilizers. CdTe nanocrystals generally have no luminescent properties until they are stabilized with 2-mercaptoethanol. However, this capping method also yields QDs with an undesirable broadband emission at higher wavelengths. Similarly, when CdTe was stabilized with thioglycerol, a broadband emission at higher wavelengths was also observed. Currently, only the capping of inorganic materials, for example, a ZnSe shell on CdTe core, can eliminate the undesirable broadband emission. Thus, there remains a need for an economic, direct aqueous synthesis route for the production of highly luminescent water-soluble nanocrystalline QDs.